Semiconductor memory device, method of controlling read preamble signal thereof, and data transmission system

ABSTRACT

A semiconductor memory device and a data transmission system that operate in synchronization with a high speed system clock without using a synchronizing circuit such as a DLL or PLL. A semiconductor memory device that operates in synchronization with a system clock provided from outside, outputs a data strobe signal from a data strobe terminal when a read command is executed, and outputs read data in synchronization with the data strobe signal, is provided with a read preamble register that specifies the length of a read preamble outputted prior to output of the read data. A memory controller gives consideration to system clock frequency and internal delay time of the semiconductor memory device, and by optimally setting the read preamble length, can perform data transmission at high speed and without missing head data even if read data output start timing of the semiconductor memory device varies.

REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of the priority ofJapanese patent application No. 2009-010251, filed on Jan. 20, 2009, thedisclosure of which is incorporated herein in its entirety by referencethereto.

TECHNICAL FIELD

The present invention relates to a semiconductor memory device, a methodof controlling a read preamble signal thereof, and a data transmissionsystem. In particular, the data transmission system relates to apreferred data transmission system that transmits data between acontroller such as a memory controller or the like, and a device to becontrolled, that is controlled by the controller, such as asemiconductor memory device or the like, that is to be controlled by thememory controller.

BACKGROUND

The increasing capacities and faster speeds of semiconductor memorydevices including dynamic RAM are remarkable. In particular, with regardto DDR SDRAM (Double Data Rate Synchronous DRAM), internal operationsare pipelined and given commands are sequentially executed bysynchronizing with a clock from outside, and in addition, high speedoperation of a system is realized so as to be able to perform datatransfer at a rate of double the clock frequency. With theabovementioned DDR SDRAM and the like, a DLL (Delay Locked Loop) circuitis used, an internal circuit is operated by synchronizing with the clockprovided from outside, and high speed data transfer is realized.

FIG. 1 is a timing diagram when a read command is executed in this DDRSDRAM. In FIG. 1, “CK” is a system clock signal provided by a memorycontroller to a CK terminal of the DDR SDRAM, and “/CK” is an invertedsignal thereof provided to a /CK terminal. In FIG. 1, the CK signal isshown by a full line, and /CK is shown by a broken line. Furthermore“Command” indicates a command given by the memory controller to the DDRSDRAM, and “Bank Add” and “Col Add” indicate a bank address and a columnaddress at that time. In addition, when a read command has beeninputted, data outputted from a DQ terminal (data terminal) of the DDRSDRAM, and a data strobe signal outputted from a DQS terminal (datastrobe terminal) are indicated by “DQ” and “DQS”. With regard to thedata strobe signal, in addition to the DQS signal, a /DQS signal that isan inverted signal thereof is outputted from a /DQS terminal, but thisis omitted in FIG. 1.

Furthermore, in addition to a standard DQS output signal, the drawingalso shows fastest and slowest timing for when output timing of the DQSsignal varies. Moreover, CAS latency is CL=6, and Additive Latency isAL=0.

In FIG. 1, first, at a rising edge at timing TO, a read command is givento the DDR SDRAM. In a standard DDR SDRAM, since a DLL is built in,phases of the system clock signals CK and /CK provided from the memorycontroller side, and the phase of a signal outputted from the DDR SDRAMto the DQS terminal, match. As described above, with the CAS latency CLof 6, since the additive latency AL is 0, after the DQS terminal outputsone cycle at low level as a read preamble in advance, it rises insynchronization with rising of the system clock of a sixth clock, T6.Thereafter, toggle operation is repeated in synchronization with thesystem clock until burst output is completed. At this time, data isoutputted in synchronization with rising and falling of the DQSterminal, from the DQ terminal. Since the DQS signal outputted from theDDR SDRAM is made synchronous by a DLL circuit with the system clocksignals CK and /CK provided from the memory controller side, there islittle phase shift. A DQS signal line is a bidirectional signal line,with cases of output as a data strobe signal from the DDR SDRAM to thememory controller side, and cases of output as a data strobe signal fromthe memory controller side to the DDR SDRAM. Therefore, where data isnot outputted in either type of case, there is a high impedance state,with an intermediate potential, according to a termination resistance.

Therefore, with regard to a DDR SDRAM specification, when data isoutputted, prior to the data output, a low level preamble signal isoutputted for a one cycle period of the CK signal. For example, as shownin FIG. 1, when the DDR SDRAM outputs read data, a read preamble signalis outputted from the DQS terminal one cycle beforehand, and this readpreamble signal is received in the memory controller, and a state ofwaiting for the read data reception is entered.

A mismatch of falling and rising edges of the DQS terminal and the /DQSterminal of the DDR SDRAM with respect to falling and rising edges ofthe CK terminal and the /CK terminal is specified as tDQSCK. With a casewhere a value of this tDQSCK is 0 (an ideal case), as tDQSCKtyp, aminimum value of variation is indicated as tDQSCKmin, and a maximumvalue is indicated as tDQSCKmax. In a normal DDR SDRAM, since a DLLcircuit is used and the phase of the DQS signal is matched with thesystem clock, there is little variation. In FIG. 1, with regard to atime-period from ts to te, the preamble signal is outputted in cases ofboth tDQSCKmin and tDQSCKmax. Therefore, in the memory controller, it ispossible to relatively easily detect the preamble signal, and in thatregard, with an average period tCKave of the system clock CK as 1875 ps,if tDQmin is −300 ps and tDQSCKmax is 300 ps, a time-period from ts tote is 1275 ps, according to Expression (1). However, in actuality, sincethere is a propagation delay in wiring from the DDR SDRAM to the memorycontroller, and a variation thereof, this type of accurate time lengthdoes not occur.

1875 ps−300 ps−300 ps=1275 ps  Expression (1)

FIG. 2 is a control flow diagram of the memory controller when a readcommand is executed. With regard to the memory controller, afteroutputting a read command in step S11, and after waiting CAS latency—onecycle in step S12, a gate detecting the DQS signal is opened in stepS13, and the read preamble signal is detected. If timing of opening thegate is made at a time at which a propagation delay is added to atime-period from the abovementioned is until te, even in a case wherethe tDQSCK varies, ideally it is possible to detect the preamble signalin one time (step S14). In a case where the preamble cannot be detected,since there is some sort of abnormality, there is a transition toabnormality processing (step S15). On the other hand, in a case wherethe preamble signal is normally detected, there is a transition todetection processing of a toggle edge (step S16). In a case where thetoggle edge could be detected, with regard to a memory read operation,since the DQS signal and the DQ signal change at the same time, the DQsignal is taken up in an intermediate time until the next edge at whichDQ signal change has become stable (step S18). An operation from thetoggle edge detection of step S16 until the DQ signal of step S18 istaken up is repeated until the data is completed, and at the completionof the data, the read operation is finished (step S19). Patent Document1 describes a circuit in which a read preamble signal is generated inthe DDR SDRAM. In the abovementioned description, it is assumedthroughout that the DDR SDRAM has a built-in DDL circuit, and that thephase of the DQS output signal matches the phase of the CK signal withgood accuracy.

On the other hand, there is a demand for reduction in power consumptionin semiconductor memory devices, not limited to devices such as notebookPCs that operate on batteries, but also in the area of servers and thelike. However, since the clock must always be operated at high speed,the DLL is a cause of power consumption increase. With regard to this,there is a description on page 37 of Non-Patent Document 1, of providinga DLL OFF mode in which the DLL is turned OFF, in a DDR3 SDRAM, which isthe latest specification of the abovementioned DDR SDRAM. According toNon-Patent Document 1, there is a description that, the DLL OFF mode isused with the CAS latency CL=6, and the DQS signal when a read operationis performed is generated from a timing signal one cycle before, withregard to a DLL ON mode.

[Patent Document 1]

JP Patent Kokai Publication No. JP-P2008-198356A

[Non-Patent Document 1]

JEDEC STANDARD DDR3 SDRAM Specification, JESD79-3B, April, 2008, JEDECSolid State Technology Association, page 37

SUMMARY

The entire disclosures of Patent Document 1 and Non-Patent Document 1are incorporated herein by reference thereto. The following analysis isgiven by the present invention. As mentioned above, there is adescription of providing a DLL OFF mode, in a DDR3 SDRAM. However, sincethere are various problems in semiconductor memory devices using a highspeed data transfer rate as in the DDR3 SDRAM, it is considered that inactuality the DLL OFF mode is hardly used. One reason for this is that,as described below, with conventional memory controllers, it isdifficult to detect a read preamble signal with suitable timing toreceive head data of read data without error.

FIG. 3 is a timing diagram in which output of the read data is startedin the DDR3 SDRAM. As described above, according to Non-Patent Document1, in the DLL OFF mode a DQS signal is controlled, with an edge of a CKsignal one cycle earlier than a CK signal specified by CAS latency, as areference. In FIG. 3, since the CAS latency CL=6 similar to FIG. 1, inthe DLL OFF mode the DQS signal is generated, with an edge of a clock T5one cycle before T6 that is the sixth cycle after receiving the readcommand, as a reference.

However, since it is not possible to adjust the phase of an internalclock so that the phase of the CK signal matches the phase of the DQSsignal, by the DLL circuit as in the DLL ON mode, the internal clock hasa phase delay with regard to the CK signal. This phase delay appears asa phase delay time tDQSCK (DLL OFF mode) of the DQS signal with respectto the CK signal, and a numerical value thereof is 2000 ps to 5000 psapproximately. That is, the tDQSCK (DLL OFF mode) is a number larger byan order of magnitude or more than a tDQSCK (DLL ON mode) when the DLLis ON as described already, and since an internal clock necessarily hasa phase delay with respect to the CK signal provided from outside, thereis a difference in that the tDQSCK (DLL OFF mode) is a positive number.As may be understood easily from FIG. 3, for a tDQSCKmin where the phasedelay is smallest, a read preamble time-period is already ended, and attiming at which a toggle operation is entered, for a tDQSCKmax where thephase delay is largest, it is still before transmission of the readpreamble.

In this type of case, there exists no timing at which the read preamblesignal can be assuredly detected even when the delay time-period of aDDR SDRAM such as timing from ts to te as shown in FIG. 1 varies. Inthis type of case, the memory controller cannot normally read the readdata, in control flow shown in FIG. 2. If an attempt is made tocorrectly receive read data output timing variations as shown in FIG. 3by the memory controller, a control flow diagram as in FIG. 4 isnecessary. In FIG. 4, processing of step S14 of FIG. 2 is replaced byloop processing of step S24 and step S21. That is, in order to correctlyread the read data in a case of tDQSCKmin, a gate must be opened beforethe tDQSCKmin starts the toggle operation (step S13), and a detectionoperation of the read preamble must be entered step S24). However, withthis timing, since the tDQSCKmax has not yet reached transmission timingof the read preamble, the preamble signal cannot be detected in thefirst detection, and with this only, abnormality processing cannot beperformed. Processing waits until it is possible to detect the preamble,and in a case where the read preamble signal cannot be detected even attiming at which the tDQSCKmax should enter the read preambletime-period, for the first time it is possible to perform abnormalityprocessing.

That is, for a controller on a side that controls the DDR SDRAM, it isconsidered necessary to perform a design change from detectionprocessing for a conventional read preamble signal. Furthermore, sincehigh speed in detection of the DQS signal is required, implementation bysoftware is difficult, and it is considered that a dedicated DLL OFFmode DQS detection circuit is necessary.

According to a first aspect of the present invention a semiconductormemory device operates in synchronization with a system clock providedfrom outside, outputs a data strobe signal from a data strobe terminalwhen a read command is executed, and outputs read data insynchronization with the data strobe signal, and includes: a readpreamble register that specifies length of the read preamble to beoutputted prior to output of the read data, and a data strobe signaloutput control unit that starts operation according to receipt of theread command, and outputs a read preamble signal of the length specifiedby the read preamble register prior to timing of starting output of theread data determined in advance, followed by outputting a data strobesignal to the data strobe terminal, from the timing of starting outputof the read data.

Furthermore, according to another aspect of the present invention, amethod of controlling a read preamble signal of a semiconductor memorydevice having a DLL selection mode mode and a DLL non-selection mode,the method includes setting the semiconductor memory device in DLLselection mode and a read preamble length of the read preamble signal.And the method further includes providing the semiconductor memorydevice with a read command, waiting a predetermined time based on theread preamble length, and detecting the read preamble signal and readdate subsequent to the read preamble signal.

In addition, according to another aspect of the present invention, adata transmission system includes: a controller; a controlled device towhich a system clock is supplied and which responds to an instructionfrom the controller to perform input and output of data with respect tothe controller; a transmission data line that connects the controllerand the controlled device and performs input and output of data; and atransmission clock line that, when the controller performs input of datafrom the controlled device, which is bidirectional and connects thecontroller and the controlled device, transmits a transmission clockgenerated by the system clock to the controller from the controlleddevice, and transmits input data in synchronization with a transmissiondata to the transmission data line. The controlled device includes apreamble length resister that determines length of a preamble signal tobe outputted to the transmission clock line prior to the controlleddevice starting transmission of data to the controller. The controllersets the preamble length register of the controlled device beforehand,and determines time from giving an instruction of data input to thecontrolled device to starting detection of the transmission clocktransmitted by the controlled device, giving consideration to frequencyof said system clock, a set value of the preamble length register, aresponse time of the controlled device with respect to an instruction ofthe controller, and variation of phase delay of a transmission clockoutputted by the controlled device with respect to the system clock.

The meritorious effects of the present invention are summarized asfollows.

According to the present invention, it is possible to provide asemiconductor memory device in which a controller that reads read datacan transmit data easily without the occurrence of missing head data.Furthermore, a data transmission system can perform transmission of dataat high speed without leakage of received data.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a timing diagram for when a read command is executed in aconventional semiconductor memory device.

FIG. 2 is a control flow diagram of a memory controller when a readoperation is performed in a conventional semiconductor memory device.

FIG. 3 is a timing diagram for when a read command is executed in a caseassuming non-usage of a DLL circuit in a conventional semiconductormemory device.

FIG. 4 is a control flow diagram of a memory controller in a caseassuming non-usage of a DLL circuit in a conventional semiconductormemory device.

FIG. 5 is a configuration diagram of an overall data transmission system(memory system) according to one exemplary embodiment of the presentinvention.

FIG. 6 is a configuration diagram of an overall semiconductor memorydevice according to one exemplary embodiment of the present invention.

FIG. 7 is a configuration diagram of a DQS (data strobe signal) outputcontrol circuit vicinity of the semiconductor memory device according toone exemplary embodiment of the present invention.

FIG. 8 is a control flow diagram of a DQS output control circuit of asemiconductor memory device according to one exemplary embodiment of thepresent invention.

FIG. 9 is a functional configuration diagram of a read preamble registerof a semiconductor memory device according to one exemplary embodimentof the present invention.

FIG. 10 is a timing diagram for a memory system (data transmissionsystem) according to one exemplary embodiment of the present invention.

PREFERRED MODES

Modes of the present invention are described making reference to thedrawings as necessary. The drawings cited in the description of themodes and reference symbols of the drawings are shown as an example ofthe modes, and are not intended to limit variations of the modes of thepresent invention thereby.

A semiconductor memory device 1 of a mode of the present invention, asshown in FIG. 6 for example, operates in synchronization with a systemclock CK provided from outside, outputs a data strobe signal DQS from adata strobe terminal 22 when a read command is executed, and outputsread data DQ in synchronization with the data strobe signal DQS, andincludes: a read preamble register 73 that specifies length of a readpreamble to be outputted prior to output of the read data, and a datastrobe signal output control unit 32 that starts operation according toreceipt of the read command, and outputs a read preamble signal of thelength specified by the read preamble register 73 prior to timing ofstarting output of the read data determined in advance, followed byoutputting a data strobe signal DQS to the data strobe terminal 22, fromthe timing of starting output of the read data. Since it is possible tochange the length of the read preamble by the read preamble register,the semiconductor memory device does not have a synchronizing circuitsuch as a DLL, PLL or the like, and even in a case where output timingof the data strobe signal varies, it is possible to lengthen the readpreamble signal length according to this variation. Therefore, it ispossible to assuredly start detection of the data strobe signal in theread preamble time-period. Furthermore, since it is possible to changethe read preamble length from a relationship of size of the variationand clock frequency, there is no risk of lengthening the read preamblemore than necessary and of lowering transmission efficiency of the data.

Furthermore, the semiconductor memory device 1 of a mode of the presentinvention, as shown in FIG. 6, further includes: a DLL circuit 24, and aDLL selection circuit 31 that switches between a DLL selection mode inwhich phase is adjusted with respect to a system clock CK using the DLLcircuit 24 and read data is transmitted, and a DLL non-selection mode inwhich read data is outputted with the system clock as reference, withoutusing the DLL circuit 24, and changes a reference of the number ofclocks of the system clock, from the read command being inputted, tostarting output of the read data, in the DLL selection mode and the DLLnon-selection mode. Therefore, the number of system clocks CK from theread command being inputted until starting output of the read data inthe DLL non-selection mode may be different from the number of clocksthereof in the DLL selection mode. In the DLL non-selection mode, sincethe phase of an internal clock generated from the system clock isnecessarily delayed, the phase of a data strobe signal generated fromthis delayed internal clock is also delayed. Therefore, it is possibleto select a preceding edge as an edge of a clock that is a reference forstarting read data output so as to compensate for this phase delay. Thedata preamble signal is outputted further preceding the timing ofstarting this read data.

Furthermore, in the semiconductor memory device 1 according to a mode ofthe present invention, as shown in FIG. 6 for example, the read preamblelength prior to the read data output is the length specified by the readpreamble register 73 when the DLL selection circuit 31 is in the DLLnon-selection mode, and is a fixed length that does not depend on thevalue of the read preamble register when the DLL selection circuit 31 isin the DLL selection mode. That is, when it is possible to select theDLL selection mode and the non-selection mode, in the DLL selectionmode, since there is little phase error of the data strobe signal withrespect to the system clock, the read preamble length may be fixed.However, in the DLL non-selection mode, when the DLL circuit is notused, since variation of phase delay of the data strobe signal becomeslarge with respect to the system clock CK signal, it is possible tochange the read preamble time-period in accordance with a relationshipbetween this variation and the clock frequency.

Furthermore, the semiconductor memory device 1 according to a mode ofthe present invention, as shown in FIG. 6 for example, further includes:the DLL circuit 24, and the DLL selection circuit 31 that switchesbetween a DLL selection mode in which the phase is adjusted with respectto the system clock CK using the DLL circuit 24 and read data isoutputted, and a DLL non-selection mode in which read data is outputtedwith the system clock CK as reference, without using the DLL circuit 24;wherein the length of the read preamble prior to read data output is thelength specified by the read preamble register when the DLL selectioncircuit 31 is in the DLL non-selection mode, and is a fixed length thatdoes not depend on the value of the read preamble register when the DLLselection circuit 31 is in the DLL selection mode.

Furthermore, in the semiconductor memory device 1 according to a mode ofthe present invention, as shown in FIG. 6 for example, the data strobesignal output control unit 32 performs toggle output of the data strobesignal DQS from the data strobe terminal 22 based on the system clock CKuntil completion of output of the read data, outputs a postamble signalafter the output of the read data is completed, and thereafter puts thedata strobe terminal in an output high impedance state, to completeoutput processing of the read data.

In addition, a method of controlling a read preamble signal of thesemiconductor memory device 1 according to a mode of the presentinvention operates in synchronization with the system clock providedfrom outside, outputs the data strobe signal DQS from the data strobeterminal 22 when the read command is executed, and outputs the read dataDQ in synchronization with the data strobe signal DQS, has a DLLselection mode in which the phase is adjusted with respect to the systemclock CK using the DLL 24 circuit and the read data is outputted, and aDLL non-selection mode in which the read data is outputted with aninternal clock as a reference, without using the DLL circuit, and theread preamble signal is outputted from the data strobe terminal 22 priorto output of the read data, wherein the read preamble signal length islengthened with respect to the system clock period of the DLLnon-selection mode more than the DLL selection mode. That is, if the DLLis OFF in the DLL non-selection mode, it is possible to reduce powerconsumption, but variation of output timing of the read data becomeslarge. However, by lengthening the read preamble signal length withrespect to the system clock period in the DLL non-selection mode, amemory controller can suitably select a starting time for detection ofthe data strobe signal. Since the read preamble signal length islengthened with respect to the system clock period in the DLLnon-selection mode, it is possible for the read preamble to berecognizable by a data reception side, without largely decreasing orchanging system clock frequency in the DLL non-selection mode withrespect to the DLL selection mode. Read data transmission speed in theDLL non-selection mode does not decrease greatly with respect to the DLLselection mode.

Furthermore, in the method of controlling a read preamble signal of thesemiconductor memory device 1 according to a mode of the presentinvention, as shown in FIG. 6 and FIG. 9 for example, the read preambleregister 73 specifying the read preamble signal length to thesemiconductor memory device 1 is provided and the read preamble signallength is controlled by a setting value of the read preamble register73. If an arrangement is such that the read preamble register can be setfrom the memory controller, it is possible to determine a suitable readpreamble length giving consideration to variation and the like of starttiming for data transmission in the semiconductor memory device 1. Ifthe read preamble length is long, it becomes easy for the memorycontroller to detect data transmission start timing of the read data,but if the read preamble length is unnecessarily long, lowering oftransmission efficiency results.

Moreover, in the method of controlling a read preamble signal of thesemiconductor memory device 1 according to a mode of the presentinvention, the larger the CAS latency, which is the number of clocks ofthe system clock from giving a read command until starting output offirst data, the longer the read preamble signal length is set. A phaseshift between the system clock and the data strobe signal becomesrelatively large as the frequency of the system clock becomes high andthe transmission rate increases. Therefore, in order that the memorycontroller correctly detects start timing of the transmission of theread data, it is desirable that the more the system clock frequency isincreased and latency is raised, the longer the read preamble length isset.

A data transmission system 50 according to a mode of the presentinvention, as shown in FIG. 5 for example, includes: a controller 2; acontrolled device 1 to which system clocks CK0 to CK3 are supplied andwhich responds to an instruction from the controller 2 to perform inputand output of data with respect to the controller 2; transmission datalines DQ0 to DQ63 that connect the controller 2 and the controlleddevice 1 and perform input and output of data; and transmission clocklines DQS0 to DQS7 that are bidirectional and that connect thecontroller 2 and the controlled device 1, and when the controller 2performs input of data from the controlled device 1, the transmissionclock lines DQS0 to DQS7 send transmission clocks generated by thesystem clocks CK0 to CK3 to the controller 2 from the controlled device1 in synchronization with a transmission data transmitted to thetransmission data lines DQ0 to DQ63; and further includes a preamblelength resister 73 that determines length of a preamble signal to beoutputted to the transmission clock lines DQS0 to DQS7 prior to thecontrolled device 1 starting transmission of data to the controller 2;wherein the controller 2 sets the preamble length register 73 of thecontrolled device beforehand, and decides a time from giving aninstruction of data input to the controlled device, to startingdetection of the transmission clock transmitted by the controlleddevice, giving consideration to a frequency of the system clock, a setvalue of the preamble length register, a response time of the controlleddevice with respect to the instruction of the controller, and avariation of phase delay of the transmission clock outputted by thecontrolled device with respect to the system clock. The semiconductormemory device can be considered to be the controlled device thatresponds to the instruction from the controller, to perform input andoutput of data with respect to the controller. Furthermore, a memorysystem configured from the memory controller and the semiconductormemory device can be considered as a data transmission system thatperforms data transmission between the controller and the controlleddevice that performs input and output of data in synchronization withthe instruction given from the controller, with system clocksynchronization. The controller starts detection of the transmissionclock at a preferable timing, giving consideration to variation of phasedelay and the like, of the transmission clocks outputted by thecontrolled device with respect to the system clock.

Furthermore, in the data transmission system 50 according to a mode ofthe present invention, as shown in FIG. 5, the controller 2 givesconsideration to the frequency of the system clocks CK0 to CK3 and thephase delay variation, sets the preamble length register 73 so thattransmission time-period of the preamble signals overlap in a case ofmaximum variation and a case of minimum variation, and even when thevariation is minimum, detection of the transmission clock transmitted bythe controlled device is started before the transmission time-period ofthe preamble signal ends. By the controller starting sensing of thetransmission clock signal at a timing at which the preamble signal isoutputted even if there is a phase shift of the transmission clockoutputted by the controlled device, it is possible to detect an initialedge of the transmission clock that is the start of data transmission,relatively easily and accurately. For example, if detection of thetransmission clock signal is started from before preamble signaltransmission, a transmission clock line has a high impedance unstablestate even if a termination resistance or the like is present. If thecontroller starts detection of voltage level of the transmission clockline in this state, there is a risk of mistakenly detecting noise as atoggle signal of the transmission clock.

Furthermore, the data transmission system 50 according to a mode of thepresent invention starts detection of the transmission clock transmittedby the controlled device after a start of a transmission time-period ofthe preamble signal even if the variation is maximum. If sensing of thepreamble signal (transmission clock line) is started after a creationtime-period of the preamble signal has started even when thetransmission clock signal phase delay variation is largest (largestdelay), there is no mistaken detection of noise when the transmissionclock line is in a high impedance state, as a transmission clock andedge.

Moreover, the data transmission system 50 according to an embodiment ofthe present invention starts detection of the transmission clock signala fixed time-period before a transmission time-period of the preamblesignal ends, in a case where variation is minimum. For example, ifactivating a gate for detecting the transmission clock increases a powerconsumption of the controller, it is preferable to delay a starting ofdetection of the transmission clock signal as much as possible. If thisis done, the above-mentioned detection starting time occurs. If thedetection starting is delayed more than as described above, a riskoccurs that an initial edge cannot be recognized. A detailed descriptionis given below according to an exemplary embodiment, making reference tothe drawings.

Example 1

FIG. 5 is a configuration diagram of an overall memory system accordingto an example of the present invention. FIG. 5 is a 64-bit parallelread-write memory system. A semiconductor memory device 1 is an 8-bitparallel read-write device, and 8 semiconductor memory devices 1 areconnected in parallel to perform 64-bit parallel reading and writing.Furthermore, in the memory system of FIG. 5 a configuration is assumedin which 2 ranks of the semiconductor memory devices 1 are mounted oneach of two DIMMs (Dual Inline Memory Module). That is, theconfiguration is of a total of 4 ranks of semiconductor memory devices,DIMM1 Rank-1, DIMM1 Rank-2, DIMM2 Rank-1, DIMM2 Rank-2, and each rankfurther includes 8 semiconductor memory devices 1. The total number ofsemiconductor memory devices 1 mounted is 4 (ranks)×8=32. In FIG. 5,only 3 semiconductor memory devices 1 among the 8 semiconductor memorydevices 1 that are connected in parallel are illustrated. Signals thatcontrol these 32 semiconductor memory devices 1 are given by a memorycontroller 2, and data input and output are performed with respect tothe memory controller.

In FIG. 5, the memory controller 2 is directly connected to eachsemiconductor memory device 1, but a PLL or buffer register may bedisposed for each DIMM between the memory controller 2 and thesemiconductor memory devices 1, and the configuration may realizesynchronization of timing with the memory controller for each DIMM, withwhat is called a Registered DIMM or a Fully Buffered DIMM (FBDIMM).Moreover, the memory controller 2 may be an LSI circuit having adedicated memory control function, and a CPU may directly controlmemory. Furthermore, the memory controller 2 may be configured of aplurality of LSI circuits.

In FIG. 5, ADR and CMD are address and command signals, and are linkedin common to the semiconductor memory devices 1 of each rank from thememory controller 2. The command signal CMD includes a row addressstrobe signal /RAS, a column address strobe signal /CAS, and a writeenable signal /WE. Signals DQ0 to DQ63 are bidirectional datainput-output signals used in read-write data transfer and the likebetween the memory controller 2 and the semiconductor memory devices 1.Since the semiconductor memory devices 1 are assumed to be 8-bitparallel input-output devices, in order to cover 64 bits of DQ0 to DQ63,8 semiconductor memory devices 1 are connected in parallel. The DQ0 toDQ63 data input-output signals are also commonly connected to each rank.Furthermore, signals DQS0 to DQS7 and signals /DQS0 to /DQS7, each beinga differential data strobe signal, are strobe signals of data to betransferred from the memory controller 2 to the semiconductor memorydevices 1 when performing a write operation, and from the semiconductormemory devices 1 to the memory controller 2 when performing a readoperation, and are bidirectional input-output signals. Since the datastrobe signals DQS and /DQS outputted by the semiconductor memorydevices 1 when performing a read operation are synchronous with a readdata change point, when used as a strobe signal on the memory controller2 side, they are used by shifting phase at timing at which a data signalcan be latched. These data strobe signals DQS0 to DQS7 and /DQS0 to/DQS7 are also commonly linked to the semiconductor memory devices ofeach rank. However, independent DQS signals and /DQS signals are linkedrespectively to the 8 semiconductor memory devices 1 that are connectedin parallel in each rank.

Furthermore, clock signals CK0 to CK3 and /CK0 to /CK3, clock enablesignals CKE0 to CKE3, chip select signals /CS0 to /CS3, and built-intermination resistance control signals ODT0 to ODT3 are signalsoutputted from the memory controller 2 to the semiconductor memorydevices 1, and separate signals are independently outputted for eachrank. The clock signals CK0 to CK3 and /CK0 to /CK3 are signals given assystem clocks to the semiconductor memory devices 1, and read-writecommands and the like given to the semiconductor memory devices 1 by thememory controller 2 are also given in synchronization with the systemclock. The clock enable signals CKE0 to CKE3 are signals that determinewhether a clock CK is valid or invalid. In a case where a CKE has a highlevel with a rising edge of the clock CK, a subsequent CK rising edge isvalid. Otherwise, the subsequent CK rising edge is invalid. With respectto the chip select signals /CS0 to /CS3, for a low level, a commandinput is valid. When /CS0 to /CS3 have a high level, a command isignored; however, operation is continued. Using the chip select signals,a command is given to a semiconductor memory device 1 of an arbitraryrank, among the semiconductor memory devices 1 of the plural ranks, andselective access is possible. Furthermore, when a value of a terminationresistance built into each of the semiconductor memory devices 1 is notinfinite, the built-in termination resistance control signals ODT0 toODT3 can perform ON-OFF control of the termination resistance.

In FIG. 5, each semiconductor memory device 1 is provided with a readpreamble register 73. As described in detail later, by each of thesemiconductor memory devices 1 being provided with the read preambleregister 73, it is possible to change the length of a read preamblesignal outputted to the data strobe signal lines DQS0 to DQS7 and /DQS0to /DQS7 prior to read data output. If it is possible to increase lengthfor a system clock period of a read preamble signal of eachsemiconductor memory device 1, even if there is variation in phase shiftwith regard to the system clock of the data strobe signals DQS0 to DQS7and /DQS0 to /DQS7 outputted by each semiconductor memory device 1 whena read command is executed, the memory controller 2 can set the readpreamble length to a length at which it is possible to detect easily andwithout error the data strobe signals DQS0 to DQS7 and /DQS 0 to /DQS7outputted by the semiconductor memory devices when a read command isexecuted.

The memory controller 2 includes a memory control unit 4 that gives acommand to a semiconductor memory device 1, sets a mode register or thelike of the semiconductor memory device 1, and performs input and outputof read-write data. Furthermore, the memory controller 2 includes a DQSsignal detection start time determination unit 5 and a preamble lengthdetermination unit 6. The preamble length determination unit 6 givesconsideration to variation of delay time of the DQS signals with regardto the system clock of the semiconductor memory device 1 when a readcommand is executed, and determines the read preamble length. Based onthe length determined by the preamble length determination unit 6, thememory control unit 4 sets the read preamble length in the read preambleregister 73 of the semiconductor memory device 1. The preamble lengthdetermination unit 6, in FIG. 3 for example, determines the length ofthe read preamble so that a time-period (for example, the time-period tsto te in FIG. 1) occurs in which the read preamble time-period oftDQSCKmax overlaps the read preamble time-period of tDQSCKmin. Values oftDQSCKmax and tDQSCKmin of the semiconductor memory device and frequencyof the system clock CK are known in advance.

Furthermore, at system start-up time or for each fixed time interval,the memory controller 2 may perform a data read test for thesemiconductor memory device 1, may measure tDQSCK values, and maydetermine the preamble length according to measured values of thetDQSCKmax and tDQSCKmin that have been thus measured. The determinedread preamble length is set in the read preamble register 73 by thememory control unit 4.

The DQS signal detection start time determination unit 5, based on theread preamble length set in the read preamble register 73, the values ofthe tDQSCKmax and tDQSCKmin, and the frequency of the system clock CK,makes a DQS signal detection start time determination so as to startdetection of the DQS signal before an initial edge (at timing te inFIG. 1) in a case of tDQSCKmin, and based on this determination thememory control unit 4 starts detection of the DQS signal. The DQS signaldetection start time is desirably after starting output of the readpreamble signal in a case of tDQSCKmax (at timing after ts in FIG. 1).If it is possible to start detection of the DQS signal between ts and tein FIG. 1, in subsequent processing it is possible to receive read datain a procedure the same as processing flow of FIG. 2, as shown in FIG.2. Moreover, the DQS signal detection start time determined by the DQSsignal detection start time determination unit 5 desirably starts at asdelayed a time as possible, within a time-period from ts to te inFIG. 1. Since power consumption increases in the time during which agate is open in order for the memory controller to detect the DQSsignal, if the detection start time is delayed, the power consumption ofthe memory controller can be decreased by a corresponding amount.

Since, among functions processed by the memory controller 2, a processrequiring real time (detection of DQS signal, reading of data, and thelike) cannot easily be carried out by software processing, a dedicatedcircuit is provided. But in the DQS signal detection start timedetermination unit 5, if time from giving a read command to starting theDQS signal detection is determined in advance, this time may be setthereafter in the memory control unit 4, so that real time processing isnot required in the DQS signal detection start time determination unit 5or the preamble length determination unit 6.

Next, FIG. 6 is an overall configuration diagram of a semiconductormemory device 1. In FIG. 6, 10 is a memory cell array, 11 is a rowdecoder that decodes a row address and drives a selected word line, 12is a sense amplifier, 13 is a column decoder that decodes a columnaddress and selects a selected bit line, 14 is a command decoder thatreceives as input a prescribed address signal and a command signal (chipselect /CS, row address strobe /RAS, column address strobe /CAS, writeenable /WE) and decodes a command, 15 is a control logic part, 16 is acolumn address buffer/burst counter, 17 is a mode register that receivesaddresses A0 to A13 and signals BA0, BA1, and BA2 for bank selection(selection of one from among 8 banks), 18 is a row address buffer, 19 isa refresh counter circuit that counts up when a refresh control signalREFC, and outputs a count output as a refresh address, 20 is a clockgenerator, 21 is a data input-output terminal (DQ terminal) forinputting and outputting read-write data to and from the outside, 24 isa DLL circuit, 25 is a data input-output unit that uses a clock given bya DLL or a clock generated from an external clock and performs input andoutput of data between the DQ terminal and the memory cell array 10.

Moreover, the mode register 17 is provided with, in addition to the readpreamble register 73, a CAS latency specification register thatspecifies CAS latency, which is omitted from FIG. 6, and a DLL selectionregister that switches between a DLL selection mode and a DLLnon-selection mode. Output of this DLL selection register is inputted toa DLL selection circuit 31, and selection is possible as to whether touse a clock whose phase is adjusted by the DLL circuit 24, as aninternal clock inputted to the DQS output control circuit 32, or to usean internal clock whose phase, which has not been adjusted by the DLLcircuit 24, is delayed, as it is, as a clock of the DQS control circuit32. When a setting is made to the DLL non-selection mode, the DLLcircuit 24 is OFF, and it is possible to reduce power consumptionconsumed by the DLL circuit 24. In addition, when a setting is made tothe DLL non-selection mode, a power supply of the DLL circuit may bearranged to automatically turn off. Furthermore, in a case where the DLLselection mode and the non-selection mode are dynamically switched,since time is required from when the DLL is put ON to a locked state inwhich the DLL is stable, in a case where the DLL non-selection mode isswitched to the DLL selection mode, first a switch is made from aDLL-OFF to a DLL-ON state, there is a waiting period until the DLL locks(for example, the CK signal is 512 clocks), and switching may take placeautomatically from the DLL non-selection mode to the DLL selection mode.

Moreover, a DQS output control circuit 32 that controls output of astrobe signal DQS outputted when data is read and a DQS output buffer 33are provided, and when a read command is executed, timing is adjusted bythe DQS output control circuit 32, and after a read preamble signalwhose length is specified by the read preamble register 73 is outputted,a DQS signal amplified by the DQS output buffer 33 is outputted from theDQS terminal 22. In addition, data is outputted from the DQ terminal 21in synchronization with output of the DQS signal. Furthermore, when awrite command is executed, a data strobe signal is received from the DQSterminal 22, and data is taken in by a data input-output unit 25 fromthe DQ terminal in synchronization with the DQS signal adjusted by theDQS input buffer 34.

FIG. 7 is a circuit block diagram of the DQS output control circuit 32and vicinity thereof. The DQS output control circuit 32 receives asupply of an internal clock from the DLL selection circuit 31, tooperate. In addition, an output signal of the read preamble register 73,a read flag not shown in the drawings, and an output signal of a CASlatency specification register are received by the DQS output controlcircuit 32; the read flag is an internal flag that is set when a readcommand is inputted, and is reset when output of the read data ends.Furthermore, an output signal of the DQS output control circuit 32 isalso a timing control signal of read data output of the datainput-output unit 25, in addition to the DQS output buffer circuit 33.

FIG. 8 is an operation flow chart of the DQS output control circuit 32.The DQS output control circuit 32 starts processing by detection of theread flag set when the read command is inputted (step S31). When settingof the read flag is detected, a count of the internal clock is started(step S32). A count of the internal clock is continued until readpreamble signal output timing, decided by the read preamble register 73prior to the read data output start timing decided in advance by the CASlatency specification register, is reached (step S33), and when theinternal clock is counted until preamble signal output timing, output ofthe preamble signal is started (step S34). The DQS output controlcircuit 32 continues the count of the internal clock, and when theoutput timing of the read data decided by the CAS latency specificationregister is reached (step S35), toggle output of the DQS signal isstarted. Together with the DQS signal toggle, the read data is outputtedfrom the DQ terminal (step S36). This toggle operation continues untilread data output of a prescribed burst length is completed (step S37).When the data output of the prescribed burst length is completed, apostamble signal is outputted (step S38), and when output of thepostamble signal is completed (step S39), the DQS terminal returns to ahigh impedance state, and output processing of the read data iscompleted (step S40).

FIG. 9 is a functional configuration diagram of the read preambleregister 73. As shown in FIG. 9, if the read preamble register 73 is aregister of a 3-bit configuration, it is possible to select a maximum of8 read preamble lengths. In FIG. 9, by a setting of the read preamblelength register, the read preamble length is from 5 selections, from 1system clock 1 period (1 tCK) to 5 periods (5 tCK). In addition, theread preamble length can be selected not for each 1 period of the systemclock but in half period units.

FIG. 10 is a timing diagram when the read command is executed in a casewhere a bit Az=0, a bit Ay=0, and a bit Ax=1 are set to the readpreamble register specification value of FIG. 9, with the sameconditions as in FIG. 3, and the read preamble length is an amount of 2system clock periods (2 tCK). Since an average period of the systemclock tCK is tCKave=1875 ps, the read preamble length tRPRE=2.0 tCK=3750ps. Furthermore, since a time difference of tDQSCKmax and tDQSCKmin istDQSCKwindow=tDQSCKmax−tDQSCKmin=3000 ps, then tRPRE>tDQSCKwindow. Ifthis relationship of tRPRE>tDQSCKwindow is satisfied, in a time oftRPRE−tDQSCKwindow only, a tDQSCKmax preamble is entered before thetDQSCKmin preamble finishes. That is, if the length of time in whichpreamble times of tDQSCKmax and tDQSCKmin overlap is tse, Expression (2)below holds.

tse=tRPRE−(tDQSCKmax−tDQSCKmin)  Expression (2)

That is, the preamble length determination unit 6 of the memorycontroller may determine the read preamble time-period tRPRE so thattse>0. In this regard, in FIG. 10, tse=3750 ps−3000 ps=750 ps.Therefore, in this time-period it is desirable that the memorycontroller 2 starts detection of the DQS signal at timing between ts andte in FIG. 10. The DQS signal detection start time determination unit 5of the memory controller 2 determines the DQS signal detection starttime so as to start detection of the DQS signal at timing after ts andup to te.

In the abovementioned exemplary embodiment, considering the memorycontroller 2 as a controller that controls data transmission, thesemiconductor memory device 1 as a controlled device that responds to aninstruction from the controller to perform input and output of data withrespect to the controller, a DQ bus as a transmission data line thatconnects the controller and the controlled device and performs input andoutput of data, a DQS signal line as a bidirectional transmission clockline that connects the controller and the controlled device and when thecontroller performs input of data from the controlled device, transmitsa transmission clock generated by the system clock to the controllerfrom the controlled device and transmits input data in synchronizationwith a transmission clock to the data transmission line, and a readpreamble register as a preamble length register that decides the lengthof the preamble signal outputted to the transmission clock line prior tothe controlled device starting transmission of the data to thecontroller, the present invention is clearly not limited to the memorysystem 50 and can be applied to a data transmission system fortransmission between a controller and a controlled device outside of thesemiconductor memory device.

An additional mode of the present invention, a method of controlling aread preamble signal of a semiconductor memory device includes:operating in synchronization with a system clock provided from outside,outputting a data strobe signal from a data strobe terminal when a readcommand is executed, and outputting read data in synchronization withthe data strobe signal, having a DLL selection mode in which phase isadjusted with respect to the system clock using a DLL circuit, to outputread data, and a DLL non-selection mode in which read data is outputtedwith an internal clock as a reference, without using the DLL circuit,and outputting the read preamble signal from the data strobe terminalprior to output of the read data; wherein length of the read preamblesignal is lengthened with respect to the period of the system clock inthe DLL non-selection mode more than in the DLL selection mode.

It should be noted that other objects, features and aspects of thepresent invention will become apparent in the entire disclosure and thatmodifications may be done without departing the gist and scope of thepresent invention as disclosed herein and claimed as appended herewith.

Also it should be noted that any combination of the disclosed and/orclaimed elements, matters and/or items may fall under the modificationsaforementioned.

1. A semiconductor memory device that operates in synchronization with asystem clock provided from outside, outputs a data strobe signal from adata strobe terminal when a read command is executed, and outputs readdata in synchronization with said data strobe signal, said semiconductormemory device comprising: a read preamble register that specifies lengthof a read preamble to be outputted prior to output of said read data;and a data strobe signal output control unit that starts operationaccording to receipt of said read command, and outputs a read preamblesignal of said length specified by said read preamble register prior totiming of starting output of said read data determined in advance,followed by outputting a data strobe signal to said data strobeterminal, from said timing of starting output of said read data.
 2. Thesemiconductor memory device according to claim 1, further comprising: aCAS latency specification register that specifies said timing ofstarting output of said read data; wherein said timing of startingoutput of said read data is determined depending on a value set in saidCAS latency specification register.
 3. The semiconductor memory deviceaccording to claim 1, comprising: a read flag that is set when said readcommand is received; wherein said data strobe signal output control unitstarts operation according to said read flag being set.
 4. Thesemiconductor memory device according to claim 1, further comprising: aDLL circuit; and a DLL selection circuit that switches between a DLLselection mode in which phase is adjusted with respect to said systemclock using said DLL circuit and read data is transmitted, and a DLLnon-selection mode in which read data is outputted with said systemclock as reference, without using said DLL circuit; wherein a number ofclocks of said system clock from said read command being inputted untilstarting output of said read data in said DLL non-selection mode isdifferent from a number of clocks thereof in said DLL selection mode. 5.The semiconductor memory device according to claim 4, wherein length ofsaid read preamble prior to said read data output is: said lengthspecified by said read preamble register when said DLL selection circuitis in said DLL non-selection mode, and a fixed length that does notdepend on a value of said read preamble register when said DLL selectioncircuit is in said DLL selection mode.
 6. The semiconductor memorydevice according to claim 1, further comprising: a DLL circuit; and aDLL selection circuit that switches between a DLL selection mode inwhich phase is adjusted with respect to said system clock using said DLLcircuit and read data is outputted, and a DLL non-selection mode inwhich read data is outputted with said system clock as reference,without using said DLL circuit; wherein length of said read preambleprior to said read data output is said length specified by said readpreamble register when said DLL selection circuit is in said DLLnon-selection mode, and a fixed length that does not depend on a valueof said read preamble register when said DLL selection circuit is insaid DLL selection mode.
 7. The semiconductor memory device according toclaim 1, wherein said data strobe signal output control unit performstoggle output of said data strobe signal from said data strobe terminalbased on said system clock until completion of output of read data,outputs a postamble signal after output of said read data is completed,and thereafter puts said data strobe terminal in an output highimpedance state to complete output processing of said read data.
 8. Thesemiconductor memory device according to claim 1, wherein said readpreamble signal is a signal of a fixed level of one of a high level anda low level.
 9. A method of controlling a read preamble signal of asemiconductor memory device including a DLL selection mode and a DLLnon-selection mode, said method comprising: setting the semiconductormemory device in DLL non-selection mode and a read preamble length ofsaid read preamble signal, and providing said semiconductor memorydevice with a read command, waiting a predetermined time based on saidread preamble length, and detecting said read preamble signal and readdata subsequent to said read preamble signal.
 10. The method ofcontrolling the read preamble signal of the semiconductor memory deviceaccording to claim 9, wherein length of said read preamble signal islengthened in said DLL non-selection mode more than in said DLLselection mode.
 11. The method of controlling the read preamble signalof the semiconductor memory device according to claim 9, wherein a readpreamble register specifying the length of said read preamble signal tosaid semiconductor memory device is provided, and the length of saidread preamble signal is controlled by a set value of said read preambleregister.
 12. The method of controlling the read preamble signal of thesemiconductor memory device according to claim 9, wherein the larger aCAS latency, which is the number of clocks of a system clock from givinga read command until starting output of initial data, the longer thelength said read preamble signal is set to.
 13. A data transmissionsystem comprising: a controller; a controlled device to which a systemclock is supplied and which responds to an instruction from saidcontroller to perform input and output of data with respect to saidcontroller; a transmission data line that connects said controller andsaid controlled device and performs input and output of data; and atransmission clock line that, when said controller performs input ofdata from said controlled device, which is bidirectional and connectssaid controller and said controlled device, transmits a transmissionclock generated by said system clock to said controller from saidcontrolled device in synchronization with a transmission datatransmitted to said transmission data line; wherein said controlleddevice comprises a preamble length resister that determines length of apreamble signal to be outputted to said transmission clock line prior tosaid controlled device starting transmission of data to said controller;wherein said controller sets said preamble length register of saidcontrolled device beforehand, and determines time from giving aninstruction of data input to said controlled device to startingdetection of said transmission clock transmitted by said controlleddevice, giving consideration to frequency of said system clock, a setvalue of said preamble length register, a response time of saidcontrolled device with respect to an instruction of said controller, andvariation of phase delay of a transmission clock outputted by saidcontrolled device with respect to said system clock.
 14. The datatransmission system according to claim 13, wherein said controller givesconsideration to the frequency of said system clock and variation ofsaid phase delay, to set said preamble length register so thattransmission time-periods of preamble signals overlap in a case wheresaid variation is maximum and a case where said variation is minimum,and even when said variation is minimum, starts detection of saidtransmission clock transmitted by said controlled device before saidpreamble signal transmission time-periods end.
 15. The data transmissionsystem according to claim 14, wherein, even if said variation ismaximum, detection of said transmission clock transmitted by saidcontrolled device is started from when a transmission time-period ofsaid preamble signal starts.
 16. The data transmission system accordingto claim 14, wherein, in a case where said variation is minimum,detection of said transmission clock is started a fixed time before thetransmission time-period of said preamble signal ends.
 17. The datatransmission system according to claim 13, wherein said controller is amemory controller; said controlled device is a semiconductor memorydevice; transmission of data performed by said controlled device to saidcontroller in response to an instruction from said controller is atransmission of read data performed by said semiconductor memory devicein response to a read command from said memory controller; and saidtransmission clock line is a signal line that transmits a data strobesignal.
 18. A semiconductor device comprising: at least one firstterminal supplied with a data strobe signal from an outside of thedevice, the data strobe signal changing to a first logic level during apreamble period of time to indicate an initiation of a data transfer andthen clocking between the first logic level and a second logic level; atleast one second terminal supplied with a data signal from the outsideof the device; a data control unit coupled to the first and secondterminals to capture the data signal in response to the clocking of thedata strobe signal; a preamble control unit producing preambleinformation indicative of the preamble period of time; and at least onethird terminal coupled to the preamble control circuit to output thepreamble information to the outside of the device.
 19. The deviceaccording to claim 18, wherein the data control unit further produces adata read command and the third terminal is further coupled to the datacontrol circuit to output the data read command to the outside of thedevice.
 20. The device according to the claim 19, wherein the preambleinformation is outputted from the third terminal prior to the data readcommand being produced, followed by being supplied with the data strobesignal and the data signal respectively at the first and secondterminals.